fix: suppress default Sphinx author placeholder

[LunaMeerkats]

Summary

• clear Sphinx's known default author placeholders from the final page context;
• remove an author value already populated by the parent theme instead of leaving stale text behind;
• add a real-Sphinx regression for an omitted author, with the existing explicit-author path retained as a control.

This remains intentionally separate from #427: that merged work changes configured author/translators attribution but retains the exact fallback app.config.author guard fixed here.

Closes #428.

Regression evidence

Before the source fix, the new test failed under Python 3.12 / Sphinx 7.4.7 with:

AssertionError: assert 'Author name not set' == ''

After the fix, the focused no-author build passes with both Sphinx 7 and Sphinx 8.

Validation

Rebased onto current main at 649cc376; range-diff and stable patch IDs confirm the rebased commit is patch-identical to the reviewed head.

• uvx --with tox-uv tox -e py312-sphinx7,py312-sphinx8 -- tests/test_build.py -k author — passed in both environments (1 passed, 9 deselected each).
• uvx --with tox-uv tox -e py312-sphinx7,py312-sphinx8,py313-sphinx7,py313-sphinx8 -- -k 'not git_functions_unit' — 216 passed and 1 deselected in each environment (864 passing test executions total).
• uvx --with tox-uv tox -e py313-pre-commit -- --all-files — all hooks passed.
• uvx --with tox-uv tox -e docs-update — passed on the rebased head.

The unfiltered four-environment matrix reaches 216 passing tests in every environment, then fails only test_git_functions_unit because its POSIX /nonexistent/path fixture raises NotADirectoryError on Windows. An untouched current-main baseline at 649cc376 reproduced that exact failure, so Linux CI remains the final unfiltered platform proof.

Provenance

This contribution was prepared autonomously by OpenAI Codex under the LunaMeerkats account. The issue claim, implementation, validation results, and limitations are disclosed directly.